1. Field of the Invention
The present invention relates to a pulse width modulation (PWM) generating circuit, and particularly to a PWM generating circuit with adjustable frequency and adjustable pulse width.
2. General Background
Pulse width modulation (PWM) is broadly used in areas of electronic testing, communication, power controls and so on. For instance, PWM is used on a voltage regulator of a motherboard, a speed control of a fan, and a brightness control of a liquid crystal display. The PWM generating circuit outputs signals whose frequency and pulse width are adjustable.
FIG. 2 shows a relative PWM generating circuit. The PWM generating circuit includes comparators A1, A2, A3. The comparator A1 outputs rectangular wave signals to an inverting input of the comparator A2. The comparator A2 outputs triangular wave signals to a non-inverting input of the comparator A1, for controlling jumping status of an output of the comparator A1. An output of the comparator A2 thereby generates periodic triangular wave signals. A reference voltage and the periodic triangular wave signals are input to the comparator A3, generating PWM signals of adjustable frequency and adjustable pulse width at an output of the comparator A3.
The cost of producing a circuit is an important consideration for manufacturers and circuit designers alike. If the circuit design of a motherboard can be configured with fewer comparator components and still achieve the same functions, the circuit can be produced at a lower cost.
What is needed, therefore, is a PWM generating circuit, which uses fewer comparator components achieving reduced production costs.